FIG. 1 shows the constitution of this kind of conventional motor control device. In the drawing, reference numeral 11 denotes a motor, reference numeral 12 denotes an inverter, reference numeral 13 denotes a direct current power source, and reference numeral 14 denotes a controller incorporated with a microcomputer. The controller 14 is equipped with a rotation control unit 15, an inverter driving unit 16, a phase current detection unit 17, and an overcurrent detection unit 18.
The motor 11 is composed of a three-phase DC brushless motor, and has a stator (not shown) including coils for three phases (a U-phase coil Uc, a V-phase coil Vc, and a W-phase coil Wc), and a rotor (not shown) including a permanent magnet. As shown in the drawing, the U-phase coil Uc, the V-phase coil Vc, and the W-phase coil Wc are connected in a star connection about a neutral point N or a delta connection.
The inverter 12 is composed of a three-phase bipolar drive inverter, and has three-phase switching elements corresponding to the coils for three phases in the motor 11, specifically, six switching elements composed of IGBT or the like (upper-phase switching elements Us, Vs, and Ws; and lower-phase switching elements Xs, Ys, and Zs); and shunt resistors Ru, Rv, and Rw. Each of shunt resistors Ru, Rv, and Rw is for detecting the voltage serving as the current flowing in each phase of the inverter 12.
The upper-phase switching element Us, the lower-phase switching element Xs, and the shunt resistor Ru are arranged in series and the both ends thereof are connected to the direct current power source 13; the upper-phase switching element Vs, the lower-phase switching element Ys, and the shunt resistor Rv are arranged in series and the both ends thereof are connected to the direct current power source 13; and the upper-phase switching element Ws, the lower-phase switching element Zs, and the shunt resistor Rw are arranged in series and the both ends thereof are connected to the direct current power source 13.
The emitter side of the upper-phase switching element Us is connected to the U-phase coil Uc of the motor 11; the emitter side of the upper-phase switching element Vs is connected to the V-phase coil Vc of the motor 11; and the emitter side of the upper-phase switching element Ws is connected to the W-phase coil Wc of the motor 11. Furthermore, the gates of the upper-phase switching elements Us, Vs, and Ws; and the gates of the lower-phase switching elements Xs, Ys, and Zs are connected to the inverter driving unit 16.
Furthermore, the lower-phase switching element Xs side of the shunt resistor Ru is connected to the phase current detection unit and the overcurrent detection unit 18; the lower-phase switching element Ys side of the shunt resistor Rv is connected to the phase current detection unit 17 and the overcurrent detection unit 18; and the lower-phase switching element Zs side of the shunt resistor Rw is connected to the phase current detection unit 17 and an overcurrent detection unit 18.
The rotation control unit 15 sends control signals for rotating the motor 11 at a prescribed rotation speed or stopping the rotation to the inverter driving unit 16 on the basis of the operation command from the operation unit (not shown).
The inverter driving unit 16 sends driving signals for turning each switching element ON or OFF to the gates of the upper-phase switching element Us, Vs, and Ws; and the gates of the lower-phase switching element Xs, Ys, and Zs of the inverter 12 on the basis of the control signals from the rotation control unit 15. The upper-phase switching element Us, Vs, and Ws; and the lower-phase switching element Xs, Ys, and Zs of the inverter 12 are turned ON or OFF in prescribed patterns by the driving signals from the inverter driving unit 16; and perform a conduction on the basis of the ON-OFF pattern, specifically, a sin wave conduction (180 degrees conduction) or a square wave conduction (120 degrees conduction) to the U-phase coil Uc, the V-phase coil Vc, and the W-phase coil Wc of the motor 11.
The phase current detection unit 17 detects currents (U-phase current Iu, V-phase current Iv, and W-phase current Iw) flowing in the U-phase coil Uc, the V-phase coil Vc, and the W-phase coil Wc of the motor 11 on the basis of the Voltages Vru, Vrv, and Vrw detected in each of the shunt resistors Ru, Rv, and Rw of the inverter 12; and sends these currents to the rotation control unit 15 as detected signals. The rotation control unit 15 detects the rotor location of the motor 11 by the detection signals (U-phase current Iu, V-phase current Iv, and W-phase current Iw) from the phase current detection unit 17.
The overcurrent detection unit 18 compares the Voltages Vru, Vrv, and Vrw detected in each of the shunt resistors Ru, Rv, and Rw (including amplified cases) of the inverter 12 with the reference voltage Vref for overcurrent determination by the comparator, and sends comparison results to the rotation control unit 15 as detected signals. The rotation control unit 15 determines presence or absence of overcurrent by the detected signals (comparison results) from the overcurrent detection unit 18, and when the presence of overcurrent is determined, the rotation control unit 15 sends the control signals for stopping the conduction by the inverter 12 to the inverter driving unit 16 for protecting the inverter 12.
Incidentally, the overcurrent protecting function performed by the rotation control unit 15 of the motor control device is assumed that the overcurrent detection unit 18 is normally operated. Therefore, even if the overcurrent occurs when an abnormality such as failure occur in the overcurrent detection unit 18, the overcurrent protecting function is not performed. Thereby, various problems such as failure of the switching element of the inverter 12, the blow-out of a power fuse, or the actuation of a circuit breaker occur by the overcurrent.
Furthermore, when the various problems occur due to the abnormality in the overcurrent detection unit 18, a long time is required for figuring out the cause. Even if the cause is figured out, a considerable amount of time and cost will be consumed for recovery treatments of the various problems.